Nitrides of elemental metals and metalloids (such as silicon) are usually very hard materials with high temperature stability, high thermal conductivity and high corrosion resistance. They are used, inter alia, as abrasives and as additives which increase the strength and hardness of metals and alloys used as cutting tools.
Nitrides are difficult to synthesise. Usually they are produced by a chemical nitration reaction. However, such chemical nitration processes are generally expensive and invariably yield mixtures of nitrides, with uncertain properties. For example, in the chemical production of tungsten nitride, the product is usually a mixture of the nitrides W.sub.2 N and WN. Similarly, molybdenum nitride production and iron nitride production usually yield mixtures of, respectively, Mo.sub.2 N+MoN, and Fe.sub.2 N+Fe.sub.4 N.
Using current chemical nitration processes, it is extremely difficult, if not impossible, to produce a composite material comprising a metal matrix material having within it a dispersed nitride phase (for example titanium and titanium nitride, Ti+TiN).
Another technique for producing metal nitrides involves a direct reaction between the metal and nitrogen at high temperature and pressure. In one implementation of this technique, metal powder is annealed in a nitrogen atmosphere at high temperatures (about 1500.degree. C.) for a long time. Usually the nitrogen is caused to flow through the metal powder throughout this process. The main disadvantages of this technique are (i) the fact that many metals do not react with nitrogen directly, even at high temperatures, so that the use of this technique is limited; and (ii) in many instances, a "plug" of the nitride is formed at the nitrogen input side of the powder and this plug restricts the flow of nitrogen, thus preventing the formation of the metal nitride throughout the metal powder. Hence this technique (as is the case with the alternative techniques of ion nitriding, salt bath nitriding, and deposition nitriding) produces metals with nitride coatings rather than bulk metal nitrides.